The aim of the paper was to determine the level of market concentration in the Polish investment fund industry from 1998 to 2018 and to identify the causes of changes. Moreover, the intention of the study was to verify a hypothesis that competition on the market can be measured by the strength and direction of the correlation between the market positions of investment fund companies (IFCs) in subsequent periods of time. The study applied measures related to the concentration curve and certain special measures as well as the correlation coefficient to identify changes in the significance of entities operating in the industry. It was established that Poland's IFC market was generally characterised by a moderate or relatively low level of market concentration. However, the level of concentration altered noticeably over time. In general, the analysed period abounded in external, market and law-related factors that could be viewed as determinants of market concentration. Furthermore, it was found that, despite many new entities entering the market and the constantly changing attractiveness of products, the investment fund industry remained ossified, mainly due to the dominant role of some large and experienced IFCs.